The resolution of optical projections imaging is proportional to the imaging wavelength λ and inversely proportional to the numerical aperture (NA) of the imaging lens. The numerical aperture is the product of the refractive index n of the light propagation medium between the imaging lens and the image and sine of the half aperture angle of the imaging lens sinθ. Presently, λ is reduced to 193 nm and development work is ongoing for 153 nm while sinθ is approaching 0.9 in many roadmaps. To further reduce λ calls for vacuum and reflective optical systems such as in the case of 13.4 nm EUV imaging. Immersion lithography, which employs a high refractive index fluid between the last surface of the imaging device and the first surface on a wafer or substrate, offers a means to increase the numerical aperture and to reduce the wavelength without concern of the physical limitations.
Immersion microlithography is a known technique for improving the resolution in optical microscope. A drop of high index fluid is placed between the front surface of the microscopic objective lens and the observed sample. Immersion lithography also uses a high index fluid between the front surface (or the last surface) of the imaging lens and the first surface on a wafer or substrate. However, simply putting a drop of fluid between these two surfaces is not sufficient for modern projection mask aligner. Many manufacturing problems have to be overcome.
It is therefore an object of the present invention to provide an apparatus for immersion lithography that does not have the drawbacks or shortcomings of the conventional photolithography apparatus.
It is another object of the present invention to provide an apparatus for immersion lithography that does not have the photoresist outgassing problem.
It is a further object of the present invention to provide an apparatus for immersion lithography that does not have the wafer overheating problem.
It is another further object of the present invention to provide an apparatus for immersion lithography that does not have the particle contamination problem.
It is still another object of the present invention to provide an apparatus for immersion lithography that includes an imaging lens, a wafer and a fluid filling a gap formed between the imaging lens and the wafer.
It is yet another object of the present invention to provide an apparatus for immersion lithography wherein a fluid having a refractive index between about 1.0 and about 2.0 is used to fill a gap formed in-between an imaging lens and a wafer.
It is still another further object of the present invention to provide a method for conducting immersion lithography by flowing a fluid through a gap formed in-between an imaging lens and a wafer.